1. Field of the Invention
The present invention relates to a method and to an apparatus for the discharge of whitewater that is accumulating inside the loop of a continuous dewatering wire in a former of a paper machine.
2. Description of the Related Art
A method and apparatus for the discharge of whitewater that is accumulating inside the loop of a continuous dewatering wire in a former of a paper machine are known, for example, from prior art documents EP 0 258 918 B1, U.S. Pat. No. 4,895,623, AT-PS 293 855 and U.S. Pat. No. 2,893,486. Hitherto it was generally customary to remove the whitewater that is the result of dewatering in a wire loop from the wire area, together with the air that is transported along with it, through an open trough and to suck the air through exhaust connections from the wire interior and the trough area. This has the disadvantage of a relatively large space requirement outside the machine frame for trough placement and the installation of separators.
The present invention creates an improved method, as well as an improved apparatus for the discharge of whitewater, whereby the previously mentioned disadvantages are removed.
The present invention provides a method for discharging the whitewater from inside the loop of a continuous dewatering wire of a former, specifically a twin wire former in a paper machine. The whitewater is collected by a collection tank, which is located within the loop. The collected whitewater is separated from the air, still inside the loop, by suctioning off of the area above the collection tank and the whitewater is removed from the collection tank through at least one channel that is connected to the collection tank and is located below its water level.
The accumulating whitewater volume is already being separated from the air inside the wire loop, or the whitewater pan to such an extent that it can be discharged from the machine area through a pipeline which is completely filled with whitewater. The previous space requirement outside the machine frame for trough placement and for the installation of separators is thereby clearly reduced. This results in savings in channels and associated building space requirement. Accordingly, possibilities of standard layouts are created, since the danger of a possible interference with the building is reduced to a minimum. The hitherto required separators adjacent to the paper machine are no longer needed. Another benefit is a simplified frame, since openings in the frame are no longer required for the purpose of whitewater drainage. These advantages are especially positive in twin wire formers.
Preferably a collection tank having as large an area as possible is utilized. The collection tank effectively extends to the two frame sides on the operator and drive side and is presently formed by these two frame sides and the corresponding tie-bars.
According to a preferred practical embodiment of the method in accordance with the present invention, the collection tank is equipped with an at least essentially air-tight hood and this hood is evacuated by an exhaust system, to separate the air from the whitewater that is collected in the collection tank. Appropriately, a hood is used that has no openings along the sides that extend transversely to the direction of travel of the machine. The only openings to the atmosphere are the necessary openings on the relevant dewatering elements on the face of the paper machine.
The whitewater can be stored in the collection tank so that the water level, which is above the channel, is maintained in the collection tank and the whitewater can flow out of the wire area from below this water level.
The flow speed of the whitewater in the exhaust area, or in the area of the collection tank is preferably less than approximately 1 m/s. Specifically, it can be less than approximately 0.5 m/s and preferably less than approximately 0.2 m/s. Such relatively low speeds result in a better exhaust level.
The length of the exhaust area, viewed in direction of machine travel, is longer than approximately 0.5 m. Specifically, it can be longer than approximately 1 m and preferably longer than approximately 2 m.
The length of the exhaust area, viewed in direction of machine travel, can at least essentially be equal to the extent of the collection tank, viewed in direction of machine travel.
According to a preferred practical embodiment of the method according to the present invention the channel which is connected to the collection tank can be open or closed on top. The water level in this channel, which is preferably at least essentially totally filled with whitewater, is lower than the water level in the collection tank.
The whitewater can be discharged laterally from the wire area through the channel which is connected to the collection tank. At least one pipeline may be connected laterally to this channel outside the wire area, through which the whitewater is supplied to a whitewater receptacle located outside the wire area and the former area. The whitewater can then be re-introduced to the process through this whitewater receptacle.
It is advantageous if the inside height or the inside diameter of the pipeline that is at least essentially completely filled with whitewater, is at least essentially identical to the inside height of the channel that is connected to the collection tank.
According to an effective arrangement of the method according to the present invention, the water level in the whitewater reservoir is higher than the water level in the pipeline and in the channel that is connected to the collection tank.
Consistent with another advantageous arrangement of the method according to the present invention at least one channel that is preferably open at the top is connected laterally and outside the wire area to the channel which is connected with the collection tank. The whitewater is supplied through this channel to a whitewater receptacle located outside the wire area and former area. The whitewater can then be re-introduced to the process through this whitewater receptacle.
According to an effective practical arrangement the water level in the channel which is connected to the collection tank is lower than the water levels in the collection tank and the whitewater receptacle.
In certain instances it is also advantageous if the water level in the collection tank is higher than the water level in the whitewater receptacle, and if the water level in this whitewater receptacle is higher than the water level in the channel which is connected to the collection tank.
In principle it is also possible that the water level in the whitewater receptacle is lower than the water level in the collection tank, whereby the level differential is specifically larger than 100 mm and preferably larger than 300 mm.
In addition there is the possibility that outside the wire area at least one pipeline is connected laterally to the channel that is connected with the collection reservoir, through which the whitewater is re-circulated back into the whitewater infeed without passing through a whitewater receptacle. This creates a spatial, as well as a cost advantage in contrast to the presence of a whitewater receptacle through which the whitewater would pass.
It is also advantageous if the flow speed in the channel is faster than 1.2 m/s and preferably faster than 2.4 m/s, whereby its cross-sectional area is smaller than 2 m2 and preferably smaller than 1 m2.
The flow speed in the channel connected to the collection tank through which the whitewater can initially be discharged in transverse direction from the wire area can be adjusted by selecting the difference between the water level in the collection tank and the water level in the whitewater receptacle. Accordingly, arbitrary whitewater quantities can be discharged. This was not possible with previously available devices having open gradients. Since the flow speed in the channel connected to the collection tank is relatively fast, relatively small cross-sectional areas result for the side openings.
The apparatus in accordance with the present invention comprises a collection tank located inside the wire loop. The tank is equipped with an at least essentially airtight hood which can be evacuated through an exhaust system, in order to separate the collected whitewater while still within the wire loop from the air. It is further equipped with at least one channel, connected to the collection tank and located below the tank""s water level, for discharging the whitewater from the collection tank.